Experiments with DrawMeshInstanced and DrawMeshInstancedIndirect for drawing grass (over terrain)

DrawGrass.cs

// https://twitter.com/Cyanilux/status/1396848736022802435

// Requires a specific shader to read the _PerInstanceData buffer at SV_InstanceID
// I use a shader made in Shader Graph, See : https://gist.github.com/Cyanilux/4046e7bf3725b8f64761bf6cf54a16eb
// Also note, there's no frustum culling involved in this example. Typically a compute shader is used for this.

using UnityEngine;

public class DrawGrass : MonoBehaviour {

	public int count; // If DrawMeshInstanced is used, this is limited to a max of 1023. No limit for DrawMeshInstancedIndirect
	public float range; // (terrain width)

	public Terrain terrain;

	public Mesh mesh;
	public Material material;

	private Matrix4x4[] matrices;
	private MaterialPropertyBlock MPB;
	private UnityEngine.Rendering.ShadowCastingMode shadowCasting = UnityEngine.Rendering.ShadowCastingMode.Off;
	private bool receiveShadows = true;

	private Bounds bounds;
	private ComputeBuffer instancesBuffer;
	private ComputeBuffer argsBuffer;

	private struct InstanceData {
		public Matrix4x4 matrix;

		public static int Size() {
			return sizeof(float) * 4 * 4;
		}
	}

	public void OnEnable() {
		MPB = new MaterialPropertyBlock();

		/*
		If using DrawMeshInstanced,
		matrices = new Matrix4x4[count];
		float range = 5f;
		for (int i = 0; i < count; i++) {
			float height = Random.Range(0.5f, 1.2f);
			Vector3 position = new Vector3(Random.Range(-range, range), height * 0.5f, Random.Range(-range, range));
			Quaternion rotation = Quaternion.Euler(0, Random.Range(0f, 360f), 0);
			Vector3 scale = new Vector3(1, height, 1);

			matrices[i] = Matrix4x4.TRS(position, rotation, scale);
		}
		*/

		// If using DrawMeshInstancedIndirect,
		bounds = new Bounds(transform.position, new Vector3(101, 1, 101));
		InitializeBuffers();
	}

	private void InitializeBuffers() {
		// Args
		uint[] args = new uint[5] { 0, 0, 0, 0, 0 };
		args[0] = (uint)mesh.GetIndexCount(0);
		args[1] = (uint)count;
		args[2] = (uint)mesh.GetIndexStart(0);
		args[3] = (uint)mesh.GetBaseVertex(0);
		argsBuffer = new ComputeBuffer(1, args.Length * sizeof(uint), ComputeBufferType.IndirectArguments);
		argsBuffer.SetData(args);

		// Instances
		InstanceData[] instances = new InstanceData[count];
		for (int i = 0; i < count; i++) {
			InstanceData data = new InstanceData();

			//Vector3 position = new Vector3(Random.Range(-range, range), 0, Random.Range(-range, range));
			Vector3 position = new Vector3(Random.Range(0, range), 0, Random.Range(0, range));

			float height = Random.Range(0.3f, 0.9f);
			float y = terrain.SampleHeight(position);

			//position.y = (height - 1) * 0.5f;
			position.y = y + (height) * 0.5f;
			Quaternion rotation = Quaternion.Euler(0, Random.Range(0f, 360f), 0);
			Vector3 scale = new Vector3(1, height, 1);

			data.matrix = Matrix4x4.TRS(position, rotation, scale);
			instances[i] = data;
		}

		instancesBuffer = new ComputeBuffer(count, InstanceData.Size());
		instancesBuffer.SetData(instances);
		material.SetBuffer("_PerInstanceData", instancesBuffer);
	}

	public void Update() {
		//Graphics.DrawMeshInstanced(mesh, 0, material, matrices, count, MPB, shadowCasting, receiveShadows);

		//bounds = new Bounds(transform.position, new Vector3(101, 1, 101));
		Graphics.DrawMeshInstancedIndirect(mesh, 0, material, bounds, argsBuffer, 0, MPB, shadowCasting, receiveShadows);
	}

	private void OnDisable() {
		if (instancesBuffer != null) {
			instancesBuffer.Release();
			instancesBuffer = null;
		}
		if (argsBuffer != null) {
			argsBuffer.Release();
			argsBuffer = null;
		}
	}
}